Thermal coke formation accompanies the generation of cracked distillate and often occurs in furnace tubes of visbreakers and delayed cokers and in any location where hydrocarbon-containing feed is maintained at around 350° C. or above for sufficient time. Foulant accumulation may manifest itself as notable changes to process conditions, such as increased pressure drop and hot spot formation because of uneven flow distributions. The onset of coke formation has been investigated for many years. It is generally believed that coke is produced as a direct byproduct of sequential polymerization and condensation reactions from lightest to heaviest fractions (maltenes, asphaltenes, and coke). Furthermore, a period of time prior to coke formation, termed the coke induction period, was identified. It is this phenomenon that permits thermal cracking processes, such as visbreaking, to operate continuously, because operating severity is preferably before the end of the coke induction period, i.e., the time at temperature (severity) that a portion of feed experiences in the furnace is optimized to ensure that coke is not formed.
Surface pre-treatment such as passivation have been demonstrated to be extremely effective at inhibiting the onset of surface coke formation in refinery units such as visbreakers and cokers under thermal cracking conditions. Under current best practice guidelines, passivator technology is applied after routine decoking events to maximize the reaction with the process surface. The passivation chemistry reacts with the metal surface. More specifically, iron reacts with the passivation chemistry and smooths the surfaces within the processing equipment.
There are three main methods employed by engineers to decoke process surfaces on thermal conversion units or other heat transfer equipment—mechanical pigging, steam/air burning, and online spalling. Sometimes the most efficient process, or the process which removes most of the surface coke, has been shown to be mechanical pigging. In this method, a sponge-like material having metal studs is used in break up the coke in the presence of water. The process is typically performed semi-annually. Generally, it is after this process that current passivation procedures are thought to be most effective. However, the pigging technique requires that the whole of the heater be taken out of service, which therefore dictates a more stringent event timetable that is often at odds with coking events during normal service.
Steam/air burning uses a circulating mass of steam and air to burn coke from process surfaces. This process is generally used semi-annually. However, the method also requires that the heater be taken out of service for at least two or three days, presenting similar engineering problems and economic issues as for mechanical pigging.
The online spalling technique is unique in that the heater may remain in service during the decoking procedure, as one furnace pass at a time is treated. Essentially, the tubes are thermally shocked using a relatively cooler liquid (e.g. water) to break or flake off the coke as the metal process equipment contracts under thermal shock. Although this method is considered in general to be the least effective, recent evidence from customer sites indicate that most of the surface coke is in fact removed, with target “clean” furnace tube temperatures achieved. The key advantage of this technique is that it may be applied anytime there is an observed fouling problem, thus providing relatively more operational flexibility. The technique of online spalling allows thermal conversion unit operators to comply with refinery wide shut down and turnaround schedules.
Treatments to remove surface coke can reduce the useful life of processing equipment so it is better to passivate the surfaces to minimize coke formation. Phosphoric acid esters with or without tert-alkyl amines are conventionally used after mechanical pigging or steam/air burning. This treatment is added when the hydrocarbons being processed are introduced into the equipment after decoking as the equipment is being brought back online. Disadvantages associated with the use of phosphoric acid ester passivation chemistries include the corrosivity of phosphorus at high temperatures encountered in hydrocarbon processing equipment, the potential for loss of primary containment, and the potential for phosphorus to foul crude distillation towers requiring premature shut down and tray replacement, or to poison hydrotreater catalysts.
It would be beneficial to provide a passivation technology that is compatible with online spalling or other decoking processes, providing an extended time period between decoking events, which would ultimately improve throughput and conversion.